Sure, I can also throw a Titan at a first level party. Its not a fair fight, its specifically UNfair. But that's what CR (or its next of kin) should tell me.
Consider the Bodak (CR 8) Death Stare DC 15. So a group of 8th level PCs consider him a "Reasonable" challenge.
Lidda the rogue has a fort save of +4 (+2 class, +1 con, +1 halfling). She must roll an 11 or better (50%) to survive. Thats a 50% death chance.
Mialee the wizard has a fort save of +2 (+2 class, no con). 13 or better (60%) she has a 40% chance of survival.
Jozan the cleric has a fort save of +8 (+6 class, +2 con). He needs only a 7 (30%) so he he has a 70% chance of living.
Tordek the fighter has a fort save of +9 (+6 class, +3 con) so hee need a 6 or better (25%). He has 75% chance of living.
(note: for speed I used the NPC stat block, adjusted for race. If you want to ajdust for PC gold, give them all a +1 cloak (+5%) and call it a day).
This normal encounter has anywhere from a 25-60% chance of killing one or more members of the group on the first initiative round. Few other CR 8 monsters (even the Mind Flayer) can KILL 1 one standard action.